Preserve literals across .asUInt (backport #4148) (#4150)

* Preserve literals across .asUInt (#4148)

(cherry picked from commit d35daa26)

# Conflicts:
#	src/test/scala/chiselTests/SIntOps.scala

* Resolve backport conflicts

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Co-authored-by: Jack Koenig <koenig@sifive.com>